Mikroskopischer Ursprung der Austauschkopplung in ferromagnetischen, antiferromagnetischen Schichten

The exchange bias anisotropy i.e. the shift of a hysteresis loop of a ferromagnet along the magnetic field axis occurs due to the exchange coupling of the ferromagnet to an antiferromagnet. Its microscopic origin is still under discussion. Available experimental data and the theoretical models for this effect are summarized. The experimentally employed methods of Brillouin light scattering, molecular beam epitaxy, and SQUID-magnetometry are presented. The influence of non-magnetic defects in the antiferromagnet ("Dilution of the antiferromagnet") on the exchange bias field is investigated in the systems Co/Co1-yO and Co/Co1-xMgxO. In both cases an increase of the exchange bias field of up to a factor of 3 is found. Monte-Carlo simulations from Dr. U. Nowak and Prof. Dr. K.D. Usadel (Univ.-Duisburg) qualitatively confirm the experimental results. These investigations result in a new model for exchange bias that is consistent with many of the experimental observations: Disorder in the antiferromagnet enables the formation of antiferromagnetic domains, which then cause a spin excess in the antiferromagnet at the ferromagnetic/antiferromagnetic interface which causes exchange bias. Furthermore, the differences in the exchange bias field determined with Brillouin light scattering and magnetometry, the dependence of the ferromagnetic magnon mode width enhancement, and the role of the applied magnetic field during cooling of the exchange bias system below the Néel temperature is investigated